1. Field of the Invention:
The present invention relates to seals for gate valve liners and, more particularly, to seals located at the throat of a gate valve liner.
2. Description of the Prior Art:
Generally, a gate valve body is machined from a single piece casting. The chest and seat of the body are machined to receive a gate. The gate is milled with parallel opposed faces in close conformity with the valve body chest. Gate valves of this type must generally be replaced in toto after a period of use depending upon the type and consistency of the fluid flowing through the gate valve. To overcome the expenses attendant replacement of complete gate valves, individually replaceable chests, collars, backing strips and seats have been developed. U.S. Pat. No. 3,624,882 is representative of gate valves of this type. Herein, segregated elements forming the seat and seals are separately replaceable. U.S. Pat. No. 3,194,259 is directed to a gate valve having a replaceable unitary structure defining segregable and distinct elements such as the gate, gate housing, seats and gate actuating mechanisms. U.S. Pat. No. 3,521,667 is directed to a control valve having a seat element and supporting structure readily replaceable as a unit. Other U.S. Pats. describing related apparatus include Nos. 2,893,684, 2,942,841, 3,050,077, 3,164,363, 3,350,058, 3,356,334 and 3,749,114.
In the patents owned by the present assignee, there is described a replaceable one piece bore and chest liner which is enclosed within a pair of mirror image castings and collectively define the housing of a gate valve. A cast gate, enveloped within molded material in the manner of a protective covering, is translatable within the liner to effect opening and closing of the gate valve. The collar of the liner described in U.S. Pat. No. 4,026,517 includes an upper surface sloping outwardly and downwardly. A compression ring bears against the upper surface of the collar to bias a seal extending inwardly from the collar in circumscribed relationship with an inserted gate against the gate. Packing disposed above the compression ring and compressed by a packing gland performs a further sealing function. U.S. Pat. No. 4,051,863 describes a plurality of bead seals disposed within the throat of the collar of the liner and bearing against the gate. A compressive force is applied to the collar, which force is translated to the bead seals, upon attachment of a stuffing box. While these seals perform their sealing function adequately, a substantial amount of pressure must be applied to effect a good long term seal. Such pressure increases the frictional forces attendant raising and lowering of the gate and contributes to wear of the sealing elements.